A general blood circuit used in hemodialysis treatment is mainly configured to include an arterial blood circuit in which an arterial puncture needle is attached to a distal end thereof and a venous blood circuit in which a venous puncture needle is attached to a distal end thereof. The blood circuit is configured so that a blood purifier such as a dialyzer can be connected to each base end of the arterial blood circuit and the venous blood circuit. A peristaltic blood pump is arranged in the arterial blood circuit and the blood pump is rotated in a state where both of the arterial puncture needle and the venous puncture needle puncture a patient. In this manner, blood is collected through the arterial puncture needle and the blood is caused to flow in the arterial blood circuit and is introduced to the dialyzer. The blood purified by the dialyzer is configured to flow in the venous blood circuit and to return to a body of the patient internally via the venous puncture needle so as to perform dialysis treatment.
In addition, a negative pressure detection device (pressure detection device) which usually detects a negative pressure is connected to an upstream side from the blood pump in the arterial blood circuit. The negative pressure detection device in the related art which consists of a member which is a so-called pillow configured to include a flexible hollow member having an internal space with a predetermined capacity. If the blood flowing in the arterial blood circuit has the negative pressure, the negative pressure detection device is configured so as to be bent in a direction where a front surface portion and a rear surface portion are close to each other (for example, refer to PTL 1). Then, for example, it is possible to detect the negative pressure by bringing a probe of the negative pressure detection device into contact with the front surface portion and the rear surface portion. One example may be found in PTL 1 Japanese Unexamined Patent Application Publication No. 2003-265601, incorporated by reference herein for all purposes.